Computer simulations reveal the workings of the dynamo behind Earth's magnetic
field

Deep in Earth's interior is a dynamo that creates the planet's magnetic field--a
kind of generator driven not by spinning turbines but by swirling flows of liquid
iron. The workings of this dynamo cannot be observed, but scientists have used computer
simulations to gain powerful new insights into the operation of the "geodynamo"
and the behavior of Earth's core.

This computer model shows the region (yellow) where the fluid flow in Earth's
outer core is the greatest. The core-mantle boundary is the blue mesh; the inner
core boundary is the red mesh. Large zonal flows (eastward near the inner core and
westward near the mantle) exist on an imaginary "tangent cylinder" due
to the effects of large rotation, small fluid viscosity, and the presence of the
solid inner core within the spherical shell of the outer fluid core.
Visit Glatzmaier's Web site
for more information and computer renderings.

The first self-consistent, three-dimensional computer simulation of the geodynamo
was achieved in 1995 by professor of earth sciences Gary Glatzmaier, then at Los
Alamos National Laboratory, and Paul Roberts, professor of mathematics at UCLA. Glatzmaier,
Roberts, and their coworkers have since refined and extended their simulations, shedding
new light on the planet's inner workings.

Glatzmaier presented the group's latest findings on Sunday, February 20, at the annual
meeting of the American Association for the Advancement of Science in Washington,
D.C. His collaborators include Robert Coe, professor of earth sciences, and postdoctoral
researcher Lionel Hongre.

The Glatzmaier-Roberts model of the geodynamo is essentially a complex set of equations
describing the physics of Earth's core. Scientists had long speculated that the mechanism
behind the geomagnetic field involved the motion of the Earth's fluid outer core,
which surrounds a solid inner core. Both are composed mainly of iron. The solid inner
core is about the size of the moon and as hot as the surface of the sun.

The flow of heat from the core ultimately drives the geodynamo. "Basically,
the whole thing works because the Earth is cooling off," Glatzmaier said. The
cooling process results in fluid motions in the outer core that produce an electric
current, which, like any electric current, generates a magnetic field.

One of the initial achievements of the Glatzmaier-Roberts model of the geodynamo
was the simulation of a reversal of Earth's magnetic field, when the north and south
magnetic poles trade places. This phenomenon has occurred many times in the history
of the planet, according to paleomagnetic records preserved in rocks that show the
direction and strength of Earth's magnetism at the time the rocks formed.

"We were able to get a magnetic field generated by the model that looks a lot
like the Earth's and undergoes reversals," Glatzmaier said.

The model also predicted that the solid inner core should rotate slightly faster
than the surface of the Earth. This prediction was later supported by other researchers
using evidence from seismic waves that pass through the core.

Over the past five years, Glatzmaier and his coworkers have improved the precision
and resolution of their model, taking advantage of advances in computer capacity.
They have now run simulations spanning as much as 300,000 years and showing a pattern
of magnetic-field reversals very similar to that seen in the paleomagnetic record.

"We can run the simulation for 200,000 years and the magnetic field will be
stable for a very long time--millions of time steps for which we solve these equations.
Then within a thousand years it reverses polarity, and then it remains stable again
for another long period. We were very happy to see that, because that's also what
we see in the Earth's record," Glatzmaier said.

He noted that the reversals are not triggered by an external influence on the geodynamo.
"It is simply due to the very nonlinear, chaotic nature of the dynamo system,"
he said.

The group's most recent efforts have focused on the role of the mantle in controlling
the frequency of geomagnetic reversals. Temperature variations in the mantle, causing
an uneven pattern of heat flow from the outer core into the mantle, may affect the
fluid dynamics of the outer core. So Glatzmaier's group ran their simulation using
eight different patterns of heat flow across the core-mantle boundary.

The results, published in the October 28, 1999, issue of the journal Nature,
showed that the pattern of heat flow determined by the mantle does have a big influence
on the behavior of the geodynamo. The most Earthlike pattern of magnetic-field reversals
occurred with a relatively uniform heat-flow pattern. This suggests that scientists
may have overestimated the extent of thermal variation in the mantle, or that variations
in mantle composition may compensate for thermal variations.

"We're still far from satisfied that we have all the answers," Glatzmaier
said. "The model is a way of exploring the unknown, and it looks very promising
because the results are so much like the real magnetic field. But we have less confidence
in the details, and that's where more powerful computers will help."